Pitting corrosion of Ni-Cr-Fe alloys at open circuit potential in chloride plus thiosulfate solutions

Alloys 600 and 800 exhibited stable pitting corrosion in 1 M NaCl plus thiosulfate deaerated solutions at the open circuit potential (OCP) and room temperature. Alloy 690 exhibited only metastable pitting. Stable pitting, stable passivity or metastable pitting were identified by observing features in the OCP vs. time transients and by analysis of electrochemical impedance spectroscopy measurements. Post-mortem scanning electron and optical microscopy observations confirmed these findings. Stable pits nucleated adjacent to Ti-rich inclusions, and reached diameters above 100 µm after 50 h of OCP immersion. A prepassivation treatment inhibited this mechanism of pitting corrosion. •S2O32- promoted pitting corrosion at open circuit potential in nickel-based alloys.•S2O32--assisted pitting corrosion required reduction of S2O32- to Sads and the presence of Cl-.•Alloys 800 and 600 exhibited stable pitting and alloy 690 only metastable pitting.•Stable pits nucleated adjacent to Ti-rich inclusions.•A prepassivation treatment inhibited S2O32--assisted pitting corrosion.